Luca,
Welcome to TradBow. There is a wealth of information on this site about shooting traditional archery tackle, as well as a number of other topics.
Don’t worry about your English, as many English-speaking people can’t write any better than if they tried to write in Italian. As for stiff and weak arrows, yes, they do fly right or left, depending. For a right-handed shooter (draws with their right hand), an arrow that is too weak for the bow at that draw length will have the nock end of the arrow to the left of the point. For an arrow that is too stiff, just the opposite. You can adjust the spine by tip weight and/or shaft length. Obviously, if an arrow is too stiff, we can’t add any length to the shaft, so the only option is to try a heavier tip. For an arrow that has a weak spine, we can either try a lighter-weight tip, or cut the arrow to a shorter length. When you cut your arrow, only cut it by small increments, to keep from cutting too much off at once and ending up with either an arrow that is too short, or too stiff. If you wind up too stiff, but still at a length that you can shoot, once again, try a heavier tip. Good luck, and let us know how we can help.
Michael.
